Lysis of Escherichia coli mutants by lactose

Growth of Escherichia coli strain MM6-13 (ptsI suc lacI sup), which as a suppressor of the succinate-negative phenotype, was inhibited by lactose. Cells growing in yeast extract-tryptone-sodium chloride medium (LB broth) were lysed upon the addition of lactose. In Casamino Acids-salts medium, lactose inhibited growth, but due to the high K+ content no lysis occurred. Lysis required high levels of beta-galctosidase and lactose transport activity. MM6, the parental strain of MM6-13, has lower levels of both of these activities and was resistant to lysis under these conditions. When MM6 was grown in LB broth with exogenous cyclic adenosine monophosphate, however, beta-galactosidase and lactose transport activities were greatly increased, and lysis occurred upon the addition of lactose. Resting cells of both MM6 and MM6-13 were lysed by lactose in buffers containing suitable ions. In the presence of MG2+, lysis was enhanced by 5 mM KCl and 100 mM NaCl. Higher slat concentrations (50 mM KCl or 200 mM NaCl) provided partial protection from lysis. In the absence of Mg2+, lysis occurred without KCl. Lactose-dependent lysis occurred in buffers containing anions such as sulafte, chloride, phosphate, or citrate; however, thiocyanate or acetate protected the cells from lysis. These data indicate that both cations and anions, as well as the levels of lactose transport and beta-galactosidase activity, are important in lysis.